Solar Heating... a primer

The sun gives us energy in two forms: light and
heat. For many years, people have been using the sun's energy to make their
homes brighter and warmer. Today, we use special equipment and specially
designed homes to capture solar energy for lighting and heating.

What are solar collectors, and how do they work?

Solar collectors trap the sun's rays to produce heat. Most solar
collectors are boxes, frames, or rooms that contain these parts: (1) clear
covers that let in solar energy; (2) dark surfaces inside, called absorber
plates, that soak up heat; (3) insulation materials to prevent heat from
escaping; and (4) vents or pipes that carry the heated air or liquid from
inside the collector to where it can be used.

Covers Many clear materials can be used as
covers for solar collectors, but glass is the most common material. Glass can
be made quickly and easily. The special glass used in solar collectors resists
breaking and scratching.

When sunlight passes through glass and hits a surface inside a solar
collector, it changes into heat. Although glass allows sunlight to pass
through, it also traps the heat produced inside the collector.

AbsorbersThe heat produced inside a solar
collector is soaked up by metal sheets or containers filled with water, rocks,
or bricks that have been painted black or another dark color. These
dark-colored objects that soak up heat are called absorbers. Without
absorbers, solar heating systems would not produce enough heat to warm rooms
inside your house.

Cars with dark seats are good examples of how the absorbers in solar
collectors work. Did you ever sit on a dark car seat in shorts after the
sun had been shining on it for a long time and the windows were closed?
Ouch! When solar energy passes through the windows of a car, heat is
absorbed by the seat. If the seats were a lighter color, like yellow or
white, light would be reflected away from the seats, and less heat would
be absorbed. Dark-colored seats absorb more
heat.

Insulation Heat always tries to move from a
hotter object to a colder one. Insulation is what prevents or slows
down the movement of heat.

Because insulation prevents the heat inside a solar collector from moving
to the outside where the temperature is lower, it is an important part of any
solar collector.

Vents and Pipes When a solar collector is
working properly, the heat that it produces moves from the collector to an
area where that heat can be used. If the collector's job is to heat air, then
vents, ducts (air tubes), and fans carry the heated air from the collector to
another part of the house. If the collector's job is to heat water, then
pipes, tubes, and pumps move water from the collector to water heating or
space heating equipment.

When fans or pumps are required to move heated air or water, the heater is
called an active solar heater. If the heated air or water from the collector
moves to another part of the house naturally without fans or pumps, then the
heater is called a passive solar heater.

Sunspaces

Solar collectors come in many shapes and sizes. A home that uses a room or
another part of the building as a solar collector is called a passive solar
home.

In many cases, passive solar homes use rooms called sunspaces to
capture solar energy directly. A sunspace can be either a room that faces
south or a small structure attached to the south side of a house.

Sunspaces have a large amount of glass and large areas of dark stone or
concrete walls and floors. These materials make up the thermal mass,
which absorbs heat.

Vents placed against the back wall of a sunspace allow heated air to move
naturally into nearby rooms. At the same time, cooler air from nearby rooms
can move into the sunspaces.

Flat-Plate Collectors

Another type of solar collector is the flat-plate collector.
Flat-plate collectors look like large, flat boxes with glass covers and
dark-colored metal plates inside that absorb heat. Flat-plate collectors are
usually placed on roofs of houses where no trees or tall buildings will block
the sun's rays.

Air or a liquid, such as water, flows through flat-plate collectors and is
warmed by the heat stored in the absorber plates. The air or water heated
inside the solar collectors then heats air or water inside the house. In an
active solar air heater, a fan pushes the air heated inside the collector into
a large bin full of rocks under the house. The heat is stored there so it can
be used later. In an active solar water heater, the water heated inside the
collector is pumped through pipes into a hot water tank.

The first flat-plate collectors were installed on the roof of a house in
Los Angeles in 1909. Since then, millions of solar water and space heaters
have been installed in homes and other buildings all over the world.

Why Use Solar Heating Systems?

Today, solar heating is becoming more important than ever before. Natural
gas and oil, which are burned to heat our homes and water, are limited. As
reserves of gas and oil shrink, these fuels become more expensive. If more
people began using solar heating systems, fossil fuels such as oil and gas
would become less expensive and last longer.

Burning natural gas and oil in our heating systems also causes air
pollution. Even electric water and space heaters cause air pollution
indirectly, because coal and natural gas are burned to produce electricity in
large power plants. So if more people used solar energy to heat the air and
water in their homes, our environment would be cleaner.

Cut out a piece of cardboard the same its width and height. height and
width of the window but with four 5-inch (12.7-centimeter) flaps extending
from the top, bottom, and sides.

Apply a coat of gesso paste to one side of the cardboard. Allow the
paste to dry for 10 minutes.

After the paste has dried, paint the same the cardboard with flat black
acrylic paint. Allow the paint to dry.

Cut vent holes 3-inches (7.6-centimeters) wide by 3-inches high near the
top and bottom flaps of the cardboard.

Push thumbtacks into the cardboard around the vent holes on the inside
surface.

Weave some thin string around the thumbtacks and across the vent holes.

Cover the thumbtacks with thin strips of duct tape to prevent them from
falling out of the cardboard.

Cut enough plastic wrap to cover the vent holes and tape the plastic to
the outside (black side) of the bottom vent holes and to the inside (string
side) of the top vent holes.

Slide the cardboard inside the window frame with the black side facing
the window and the top vent holes up. (The plastic flaps should be hanging
over the vent holes.) Tape the cardboard to the window frame with masking
tape and leave an air space between the window and the cardboard.

Hint: Don't leave your collector taped to the window frame for too many
days, or the tape may pull paint off when you remove it.

On the graph paper, draw lines marking three columns, and write the
words "Time," "Intake (bottom) temperature," and "Output (top) temperature"
at the top of the columns.

Once every hour for a few hours on a cloudy day, and for a few hours on
a sunny day, hold the thermometer under the plastic flaps covering the vent
holes for 2 minutes to measure the collector's air intake (bottom) and
output (top) temperatures. Mark your temperature readings on the graph
paper.

Did your solar collector work? During what time of the day was the
collector's output temperature the highest? What was the highest output
temperature of the collector on a cloudy day? On a sunny day?

Glossary

Absorbers: dark-colored objects that soak up heat in solar
collectors.

Active solar heater: a solar water or space heating system
that moves heated air or water using pumps or fans.

This document was produced for the U.S. Department of Energy (DOE) by the
National Renewable Energy Laboratory (NREL), a DOE national laboratory. The
document was produced by the Information Services Program, under the DOE
Office of Energy Efficiency and Renewable Energy. The Energy Efficiency and
Renewable Energy Clearinghouse (EREC) is operated by NCI Information Systems,
Inc., for NREL/DOE. The statements contained herein are based on information
known to EREC and NREL at the time of printing. No recommendation or
endorsement of any product or service is implied if mentioned by EREC.

This article appears with slight modification courtesy of EREN...
the Energy Efficiency and Renewable Energy Network , a division of the U.S.
Department of Energy.

If you have comments or questions about this article, you may visit the EREN
website at http://www.eere.energy.gov or
contact them by mail, email or phone: